Self-sealing insulating material of high dielectric strength



Patented Oct. 2, 1951 HIGH DIELECTRIC STRENGTH,

Harold E. Selby, Brooklyn, N. Y., 'assignor', by mesne assignments, toBishop Manufacturing. I Corporation, Summit, N. Jr, a corporatlonnof .1.New Jersey No Drawing.

. to an improved composition in tape, sheet ormass form which hasthep'roperty-of sealing itself into a substantially homogeneous body. ofhigh electrical insulating value whenapplied to a conjected to"relatively high 'or relatively low tem-,

peratures. All of them deteriorate quickly in the presence of ozone.-For this reason,-great dimculty has been encountered in the past in'prop- .erly insulating conductors which are used for or near highvoltage lines or other equipment where corona discharge, arcing or otherozone generating conditions are present; This is equallyitrue of therubberinsulating tapes which are widely used to forma rubbery sheatharound a conductor or a connection. Such rubber tapes are convenient touse, but inasmuch as they consist principallyof unvulcanized rubber andan accelerator, they have the disadvantage-of being self-vulcanizing sothat their shelf life is relativelyshort. Also, these tapes are stickyso that the tape tends to sticktogether, often times tearing and makingits application-to the wire or other article being insulated a ratherdifiicult and messy operation. v

The present invention overcomes the disadvantages of these priorflexibleinsulating materials by providing a composition which is unaffected oververy. longperiods'of time by high concentrations'of ozone. It is. stableand thus has a very long shelf life. It isnotespecially tacky and to orbetterthanthe rubber tapes and insulating materials produced heretofore.Moreover, my new composition, when provided in tape or sheet form, hasthe highly desirable property of being self-sealing, that is, when thematerial is stretched and then wrapped around a conductor ApplicationJanuary 4, 1949, Serial No. 69,235 V GCIaims. (c1. 260- -27) 1 or otherarticle, it has a creep or memory such that it contracts and flowstogether to form -a homogeneous body whichcannot be delaminated aftersetting for a relatively short period of time. This material, uponsetting, does not show lamlnationscorresponding to the convloutions ofthe material. Thus. ifthe tape is wound around a conductor in the usualway, a continuous sheath is provided which does not have a junction,line or zone between the overlapping portions of the material. l

The properties of the material can be varied substantially, dependingupon its intended use. Thus, the material maybe made very pliable andsoft foruse under low temperature conditions wherein ordinary insulationbecomes brittle and shatters. ,On the other hand, its properties can bemodified so that it is resistant to high temperatures and it Will adhereto and remain on the conductor without any tendency to flow away or todrop from the conductor. Compositions of optimum characteristics whichare capable of withstanding a very great range of temperatures can beproduced and thereby servev efiectively from below zero .to temperatureswell above ,thus does not tend tostick together very strongly. Inaddition, it has dielectric properties equal A particularly novelcharacteristic of my. new composition is; that it discloses visually theoptimum degree of stretch to be attained before applying the material tothe article to be insulated. In the preferred type of composition, thetape changes color when itis stretched sufiiciently to produce thememory or creep efiect which renders the tape self-sealing. 7

Generally, tapes of the type embodying the present invention and havingthe most desirable properties for commercial use consist of a compatiblemixture of at least four components.

Thesecomponents are polyethylene of a relativeconsist of 35%diphenyl-paraphenylene diamine and 65% phenyl-alpha-naphthtyl-amine.

The property of resistance to deterioration by the action of ozone isbelieved to be laregly due to the fact that this composition containsvery little unsaturated material, the only unsaturated compound presentbeing isoprene. Thus, there are few linkages available for coupling withoxygen which would cause a change in or breaking down of the plastic andinsulating properties of the composition.

The functions of the various components of my new insulating compositionare believed to be as follows: The polyethylene is in the nature of aninert filler having good insulating properties. It also serves toindicate the proper degree of stretch of the finished tape or material.Polyethylene is a good insulator but it is semi-rigid when of themolecular weight preferred in the composition and it has relatively lowphysical strength.

Butyl rubber is highly compatible with polyethylene and has the functionof reducing the rigidity of the polyethylene and providing increasedstretch in the material.

Polymerized isobutylene increases the tear strength of the material andalso appears to increase the resistance of the composition to oxidation,especially oxidation by ozone. It also,

when used in the proper molecular weight, re-

duces the rubberiness produced by the butyl rubher.

The resinous tackiflers referred to above pro-.

duce a plasticizing action and cause the material to flow underpressure, thereby making the material bond together. The resinoustackifier may be any of a large group of synthetic or natural resins orderivatives thereof which are recognized for their ability to imparttackiness to resinous products. Among those resinous tackifiers thathave been used in my new composition are cumarone-indene resins, alphamethyl styrenes, phenol-formaldehyde resins, alkyd-type resins, terpeneresins, wood rosins, rosin esters, such as ester gums, hydrogenatedditerpenes, pentaerythritol abietates, hydroabietyl alcohol, dehydroabietic acid, zinc resinates, gutta percha resin derivatives and manyothers.

The best results are obtained with abietic acid derivatives of the typesreferred to above. The preferred resinous tackifier is dehydroabieticacid which has a specific optical rotation in benzene of 49.5, an acidnumber of 158 and a saponification number of 164.

The action of the polyethylene in providing an indication of stretch isnot clearly understood but it is believed to be a function of the longchain relation between the various components whereby stretching of thecomposition forces some of the polyethylene out of the mixture orsolution and thereby causes the composition to change from a greenish toa whitish color.

The proportions of the various components of my new composition can bevaried substantially while at the same time permitting the production ofan acceptable insulating material. The polyethylene, being in the natureof an inert filler, can be varied within a relatively wide range andmight even be omitted from the composition if it were not for twodrawbacks resulting from such omission. The first of these is that theresulting composition would not have any means for indicating theoptimum stretch of the material and thus it would be diflicult todetermine when the tape has been stretched enough to put it in condi- 4tion for proper sealing. The polyethylene also acts to restrict thecreep of the other materials and in this way controls the creep of thecomposition.

The other materials cooperate with the polyethylene to reduce the eilectof thermal shock on the polyethylene component. Polyethylene, alone, ifheated and then chilled abruptly, would shatter.

The butyl rubber component has the eil'ect of imparting rubberiness tothe composition and also tends to provide physical strength which islacking in the polyethylene. Thus, it butyl rubber were omitted from thecomposition, the resulting composition does not stretch at all, and evenin the presence of 5% of butyl rubber, the material is not veryresilient and is short, that is, has a tendency to break uponstretching.

when the proportion of butyl rubber is increased to above 25%, theresulting material is quite weak and is diiilcult to process and handle.

Usually, the composition contains a mixture of low molecular weightpolymerized isobutylene and a higher molecular weight polymerizedisobutylene. Here again, the proportions of these components aresusceptible to considerable variation. When the low molecular weightisobutylene polymer is absent, the material lacks tackiness and is shortand breaks easily. This condition obtains even when as much as 4% of thelow molecular weight isobutylene polymer is present.

When more than 25% isobutylene polymer (low molecular weight) ispresent, the composition becomes very tacky and flows when heated, somuch so, that a composition containing 30% or the lower molecular weightpolymer cannot be processed on a calender at all for it sticks to therolls and cannot be removed.

The higher molecular weight isobutylene polymer has a tendency to reducethe stretch of the composition but it improves the resiliency oi thecomposition. In the absence of this material, the composition is quiteweak and short, i. e., it breaks easily, and even when as much as 5% ofthis material is present, this condition still exists to some degree.When as much as 40% of the higher molecular weight isobutylene polymeris present, the material becomes very tough, does not stretch easily andhas a hard, horny appearance. The combined amounts of the isobutylenepolymers can be varied, accordingly, between about 9% and 65% of thecomposition.

The isobutylene polymers are commercially available products identifiedby the molecular weights as given above. The molecular weights of thesepolymers are determined by the Staudinar method of determining intrinsicviscosity and viscosity average molecular weight as disclosed in theStandard Inspection Laboratory Circular No. 320.01 dated February 28,1944.

The tackifier produces the tackiness and sealing effect whichcharacterizes the new composition. It the tackifier is omitted, thecomposition stretches but does not seal readily. When about 1% of thetackifier is added, the extensibility or stretch of the material isincreased substantially and the tackiness is increased sufliciently toprovide some sealing action. The maximum amount of the tackifier islimited to about 20% inasmuch as, above this proportion, the tackifierrenders the material very sticky and short so that it is difllcult' toprocess. When as much as 25% of the tackifier is present, the materialbecomes so tacky or sticky that it cannot be processed on a calenderroll to reduce it to sheet or tape iorm.

which is satisfactory for use under many conditions. However, forcommercial use and production, the best results are obtained when thecom position contains components in about the following proportions:

Per cent Polyethylene, about 20,000 molecular weight 50 Butyl rubber 13/2 Polymerized isobutylene,

100,000 to 115,000 molecular weight 22 /2 Polymerized isobutylene,

10,000 to 20,000 molecular weight Resinous tackifler 4 This compositionstretches readily but is quite tough so that it breaks only whensubjected to very strong stretching forces, it has a slightly tacky butnot sticky surface and is or a faintly greenish color when unstretched,changing to a whitish color when stretched sufllciently to render itscreep effective for sealing action. The material recovers slowly, unlikerubber, but nevertheless will return substantially to its unstretchedcondition when relieved of tension. It

is the pressure exerted by the creep of the material which apparentlycauses the material to flow together and thereby bond or cohere socompletely as to form a substantially homogeneous body.

The composition is susceptible to further modification. Thus, forexample, a single isobutylene polymer can be used instead of theisobutylene polymers described in the specific example and the examplesgiven above. The average unit molecular weight of this single polymershould be about the same as the average of the combined unit molecularweights of the two isobutylene polymers. Thus, for example, anisobutylene polymer having a molecular weight of about 60,- 000 to75,000 can be substituted in an equal amount by weight for the combinedamounts of the two isobutylene polymers referred to in the specificexample. The ultimate result is about the same for it appears that thedegree of polymerization of this material controls its strengthimparting and resilient properties. From the preceding description oftypical compositions embodying the present invention, it will beapparent that I have provided a composition which has the stabilityunder variations of temperature and in the presence of ozone which islacking from the prior rubber tapes and rubbery insulating materials. Mynew composition is useful under conditions where the prior rubberyinsulating materials are not satisfactory. Moreover, it has theadditional property of improved appearance due to its color. It providesa stronger bond between the various laminations of the material and ithas a far greater shelf life than prior insulating tapes and the like.

As indicated above, the material is susceptible to considerablemodification in the proportions of its several components, and,therefore, the example given above should be considered as illustrativeof the invention and not as limiting the scope of the following claims.

I claim:

1. An insulating composition of high dielectric strength which uponbeing stretched and disposed ln overlapping laminationsflows togetherinto a homogeneous body. consisting essentially of a normally semi-rigidpolyethylene having electrical insulating properties, the amount ofpolyethylene being up to 50%, between about 5% and of asulfur-vulcanizable rubbery copolymer ofisobutylene and a dioleiin inapproximately the proportions of 98:2, between about 9% and 65% of atleast one polyisobutylene having an average unit molecular weight.between about 60,000 and 75,000 and between about 1% and 20% of aresinous tackifler for imparting flow and cohesiveness to saidcomposition to? cause i said laminations to flow together.

2. An insulating composition of high dielectric strength which uponbeing stretched and disposed in overlapping laminations flows togetherinto a homogeneous body consisting essentially of polyethylene having amolecular weight of about 20,- 000, the amount of polyethylene being upto 50%, between about 5% and 20% of a sulfur-vulcanizable rubberycopolymer of isobutylene and a diolefin in approximately the proportionsof 98:2, between about 9% and 65% of polyisobutylene polymer having anaverage molecular weight between about 60,000 and 75,000 and betweenabout 1% and 20% of a resinous tackifler of the class consisting ofrosin, rosin esters, dehydroabietic acid, zinc resinate and hydroabietylalcohol.

3. An insulating composition of high dielectric strength which uponbeing stretched and disposed in overlapping laminations flows togetherinto a homogeneous body consisting essentially of about 50% polyethylenehaving a molecular weight of about 20,000, about 13.5% of asulfurvulcanizable rubbery copolymer of isobutylene and a dioiefln inapproximately the proportions of 98:2, about 22.5% of a polyisobutylenehaving a molecular weight between about 100,000 and 115,000, about 10%of a polyisobutylene having a molecular weight between about 10,000 and20,- 000 and about 4% of a resinous tackifler for imparting flow andcohesiveness to said composition to cause said laminations to flowtogether, said composition having the property of changing color uponstretching to indicate the proper degree of stretch before laminating.

4. An insulating tape having high dielectric strength, said tape beingstretchable, resilient, changeable in color upon stretching and flowableinto a homogeneous mass upon stretching and winding into a plurality ofoverlying convolutions, said tape consisting essentially of a normallysemi-rigid polyethylene, the amount of polyethylene being up to 50%,about 5% to 25% of a sulfur-vulcanizable rubbery copolymer ofisobutylene and a diolefin in approximately the proportions of 98:2 forimparting resiliency to said polyethylene, about 9% to 65% of at leastone polyisobutylene having an average unit molecular weight betweenabout 60,000 and 75,000 for controlling the resiliency of thecomposition, and about 1% to 20% of a resinous tackifler for increasingthe tackiness and flowability of the composition to cause thelaminations to bond and flow together.

5. An insulating compositin of high dielectric strength in sheet formwhich upon being stretched and disposed in overlapping laminations flowstogether into a homogeneous body, consisting essentially of about 50%polyethylene having a molecular weight of about 20,000 about 13.5% of asulfur-vulcanizable rubbery copolymer of isobutylene and a diolefin inapproximately the proportions of 98:2, about 22.5% of a polyisobutylenehaving a molecular weight between about 100,000 and 115,000, about 10%of polyisobutylene having a molecular weight between about 10,000 and20,000 and about 4% of a resinous tackifler of the class consisting ofrosin, rosin esters. dehydroabietic acid, zinc reslnate, andhydroabietyl alcohol for imparting flow and cohesiveness to saidcomposition to cause said laminations to flow together. said compositionhaving the property oi changing color upon polyisobutylene having amolecular weight between about 10,000 and 20,000, and about 4% ofdehydroabietic acid, said composition having the property of changingcolor upon stretching to indicate the proper degree of stretch beforelamihating.-

HAROLD E. SELBY.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,383,839 Beekley Aug. 28, 19452,419,304 Worth et al Apr. 22, 1947 2,451,865 O'Brien Oct. 19, 19482,462,977 Kitchin et al Mar. 1, 1949 OTHER REFERENCES Flory. pages 372,373, 379-382 Jour. Am. Chem. 800., March, 1943.

1. AN INSULATING COMPOSITION OF HIGH DIELECTRIC STRENGTH WHICH UPONBEING STRETCHED AND DISPOSED IN OVERLAPPING LAMINATIONS FLOWS TOGETHERINTO A HOMOGENEOUS BODY, CONSISTING ESSENTIALLY OF A NORMALLY SEMI-RIGIDPOLYETHYLENE HAVING ELECTRICAL INSULATING PROPERTIES, THE AMOUNT OFPOLYETHYLENE BEING UP TO 50%, BETWEEN ABOUT 5% AND 2/% OF ASULFUR-VULCANIZABLE RUBBERY COPOLYMER OF ISOBUTYLENE AND A DIOLEFIN INAPPROXIMATELY THE PROPORTIONS OF 98D2, BETWEEN ABOUT 9% AND 65% OF ATLEAST ONE POLYISOBUTYLENE HAVING AN AVERAGE UNIT MOLECULAR WEIGHTBETWEEN ABOUT 60,000 AND 75,000 AND BETWEEN ABOUT 1% AND 20% OF ARESINOUS TACKIFIER FOR IMPARTING FLOW AND COHESIVENESS TO SAIDCOMPOSITION OT CAUSE SAID LAMINATIONS TO FLOW TOGETHER.